1. Field of the Invention
The present invention relates to a method and apparatus for correcting rotational errors during inspection of two flat objects, more particularly, semiconductor fabrication masks.
2. Description of the Prior Art
In order to fabricate semiconductor devices, a mask or reticle is used. The mask or reticle has a light-transmitting portion and a light-shielding portion which constitute a circuit pattern to be exposed on a wafer. The mask and reticle must be formed with a desired circuit pattern and with desired precision. An inspection apparatus is required to compare the inspected mask and reticle with a reference mask and reticle before being used to fabricate semiconductor devices.
A conventional mask inspection apparatus is known wherein the positions of points of a reference mask having a plurality of circuit patterns are compared with the positions of corresponding points of a mask to be inspected. The mask to be inspected has the same circuit patterns as those in the reference mask in one-to-one correspondence. In a mask inspection apparatus of this type, the reference mask is placed parallel to the mask to be inspected. The circuit patterns of these masks are enlarged by a pair of objective lenses, and the resultant mask images (i.e., the enlarged pattern images of the masks) are superposed and observed in the same field of view so that pattern error of the mask to be inspected can be detected. For this purpose, the mask inspection apparatus has a stage for simultaneously moving the two masks in a two-dimensional manner. A first fine adjustment table is rotatably mounted on one side of an upper surface of the stage and supports and rotates the reference mask. A second fine adjustment table is rotatably mounted on the other side of the upper surface of the stage and supports and rotates the mask to be inspected.
By rotating the first and second fine adjustment tables, the reference mask and the mask to be inspected are aligned such that an optical positional relationship between the reference mask and a corresponding objective lens corresponds to that between the mask to be inspected and a corresponding objective lens. Two-dimensional (x,y) alignment errors (translational errors) can be easily eliminated by movement of two linked plane parallel plates of a photoelectric micrometer. However, a rotational error must be eliminated by manually rotating the first and second adjustment tables. If the rotational error is not eliminated, it appears as errors of measured values of the relative positions of respective points of the masks. Therefore, conventionally, a high degree of skill is required for physically aligning the two masks to eliminate rotational alignment errors, thus increasing the alignment time.